The technique of reflow soldering electrical components or devices on a circuit board is well known. Generally, one or more electrical leads from a device are placed on top of one or more solder pads located on a circuit board. The entire circuit board, or the area immediately surrounding a particular device, is then heated until the solder comprising the interconnect between the leads and the circuit pads changes phase from solid to liquid or melts. When the solder melts the device leads and the underlying circuit pad are wetted, thereby forming a solder joint. Heating is then stopped, and the circuit board and solder joint are cooled or allowed to cool until the solder solidifies, at which point the device is fully attached. Device removal is merely a reversal of the above steps. To wit, the solder joint is heated until the solder touching the lead is heated to a temperature above its melting point, at which time the lead may be easily withdrawn from the solder, and the device removed from the circuit board. Delay or failure to detect solder reflow and subsequent temperature increases, results in excessive or unduly prolonged heating of the solder, and causes timing inefficiency, equipment damage, and poor process results.
For optimum wetting between the materials involved, the lead/pad interface and solder should be heated to a temperature above the melting point of the solder. Therefore, after reflow is sensed, the temperature must be monitored to establish that a target temperature above the melting point of the solder has been reached. In the case of eutectic or near-eutectic tin/lead solder, a temperature approximately 30.degree. C. above the melting point is desirable. During device removal a temperature 30.degree. C. above the solder's melting point allows the solder to reach a lower viscosity so that excess solder is not removed from the circuit pads by adhering to the leads of the device as the device is removed from the circuit during a replacement operation. In prior art methods, only solder temperature, not the phase change phenomena itself, has been monitored to determine when the correct temperature has been reached.
Generally unsuccessful attempts have been made to use remote or non-contact infrared sensors for the measurement of the temperature of an object within the sensor's field of view by detecting infrared emissions from the object. In the case of soldered electronic devices, the sensor is directed at a solder joint to detect temperature. Temperature monitoring with an infrared sensor and other methods of detecting and monitoring solder, such as detecting a change in surface reflectance as part of a soldering technique, are presented in U.S. Pat. Nos. 4,657,169 to Dostoomian et al.; 4,696,104 to Vanzetti et al.; and 4,696,101 to Vanzetti et al. These prior art methods were developed to monitor solder phase changes for relatively large solder joints found on typical J-lead chip carriers and leadless chip carriers (LCC). However, the inaccuracy and unreliability of these techniques has engendered little interest in the electronics industry.
FIG. 1 is a side view of a device 10 having a 50 mil pitch J-lead 12. For a J-lead 12 of this width, an electrical connection to a substrate 14 is made with a solder joint 16 that is 0.030 to 0.035 inches wide. FIG. 2 is an end view of FIG. 1 which further illustrates the general nature of the solder joint 16. With a J-lead 12 equipped device 10, the solder wicks up the J-lead 12 and around the perimeter of the lead 22, thereby presenting a substantial amount of solder for a sensor to view. This connection technique produces a viewing surface or solder joint 16 of approximately 4.times.10.sup.-4 in.sup.2.
FIG. 3 is a side view of a device 10 having 50 mil pitch leadless chip carrier leads 18. As with the J-lead 12, the LCC is electrically connected to the substrate 14 with a series of solder joints 16 that are typically 0.030 to 0.035 inches wide. The solder joint 16 extends vertically from the surface of the substrate 14 up the metalized castellation 18 of the device 10 and forms a relatively large bulbous type solder joint 16 when heated. FIG. 4 is an end view of FIG. 3 which further illustrates the general nature of the solder joint 16. The solder joint 16 of the LCC device 10 presents an even larger solder volume for sensor viewing than the J-lead 12, the LCC solder joint 16 presenting a surface of approximately 9.times.10.sup.-4 in.sup.2.
While an infrared sensor may function properly in wide or coarse pitch applications, fine pitch devices having a lead width in the range of 0.015 to 0.002 inches wide are more becoming important and form a necessary part of many new designs. Fine pitch devices typically present a reduced volume and reduced surface, a scant 1.times.10.sup.-4 in.sup.2 for viewing a 25 mil pitch device, and correspondingly less for a smaller lead width. As lead pitch gets finer, there is less and less distance between center lines of adjacent leads and the viewing area is correspondingly smaller. As the solder volume gets smaller, the change in rate of temperature rise, which is interpreted to detect phase change, is affected by the solder being heat sunk by adjacent masses. Eventually, the diminutive volume of solder is so masked by heat sinking that phase changes are not detectable.
Accordingly, prior art non-contact measurement systems are of little or no use with fine pitch devices due to limitations of field of view, volume (therefore mass) of solder, and state of the art sensors. Specifically, in non-contact detection of infrared energy the amount of solder in the field of view of the sensor appears quite small in relation to other items in the field of view, such as metal leads, the device package, and the circuit substrate. Therefore, the infrared emission from the solder joint is small in comparison to the other infrared emissions received by the sensor. This multiplicity of strong emissions masks the relatively weak signal from the solder joint at which the sensor is directed. Thus, the insufficiently restricted field of view of the sensor, or its inability to discriminate between objects in its field of view, results in its inability to sense anomalies in the rate of heating of solder materials.
In FIG. 5 for example, a fine pitch device 20 having "gull wing" leads 22, only has a thin coating (if any) of solder on top of each lead 22. This creates problems in sensing the anomaly of solder melting because the thin layer of solder more specifically represents the temperature of the underlying metal of the lead 22. Furthermore, the solder develops intermetallics with the base metal of the lead 22 so that the solder's melting point is no longer that of the solder originally deposited on the substrate. FIG. 6 is an end view of the solder joint 16 of FIG. 5, and FIG. 7 is a top view of the same solder joint 16, both views illustrating the nature of the solder joint 16.
The reduction in viewing surface presented to an infrared sensor in the fine pitch device 20 virtually eliminates the observability of the change in emissivity, reflectance, and shape of the solder while it melts using prior art techniques and devices. Even if the field of view is reduced to a 0.001 inch spot on the solder joint, the decreased solder volume used with fine pitch devices causes the solder temperature to be effectively masked by the surrounding thermal masses that are in intimate contact with the solder.
Ideally, a monitoring sensor used in the process would instantly detect and continue to accurately monitor the phase change event itself, and use that information to trigger subsequent events and calibrate subsequent measurements. The present invention fulfills the above need, and overcomes present sensing difficulties by enhancing the emissions from the area of the solder joint and by enhancing a sensor's ability to discriminate between thermal sources.